1. Field of the Invention
The invention relates to lighting troffers and, more particularly, to indirect lighting troffers that are well-suited for use with solid state lighting sources, such as light emitting diodes (LEDs).
2. Description of the Related Art
Troffer-style fixtures are ubiquitous in commercial office and industrial spaces throughout the world. In many instances these troffers house elongated fluorescent light bulbs that span the length of the troffer. Troffers may be mounted to or suspended from ceilings. Often the troffer may be recessed into the ceiling, with the back side of the troffer protruding into the plenum area above the ceiling. Typically, elements of the troffer on the back side dissipate heat generated by the light source into the plenum where air can be circulated to facilitate the cooling mechanism. U.S. Pat. No. 5,823,663 to Bell, et al. and U.S. Pat. No. 6,210,025 to Schmidt, et al. are examples of typical troffer-style fixtures.
More recently, with the advent of efficient solid state lighting sources, these troffers have been used with LEDs, for example. LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with approximately ninety percent of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs by a factor of about 10, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color reproduction. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in their LED lights being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.
Other LED components or lamps have been developed that comprise an array of multiple LED packages mounted to a printed circuit board (PCB), substrate or submount. The array of LED packages can comprise groups of LED packages emitting different colors, and specular or other reflector systems to reflect light emitted by the LED chips. Some of these LED components are arranged to produce a white light combination of the light emitted by the different LED chips.
In order to generate a desired output color, it is sometimes necessary to mix colors of light which are more easily produced using common semiconductor systems. Of particular interest is the generation of white light for use in everyday lighting applications. Conventional LEDs cannot generate white light from their active layers; it must be produced from a combination of other colors. For example, blue emitting LEDs have been used to generate white light by surrounding the blue LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). The surrounding phosphor material “downconverts” some of the blue light, changing it to yellow light. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to yield white light.
In another known approach, light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes. Indeed, many other color combinations have been used to generate white light.
Because of the physical arrangement of the various source elements, multicolor sources often cast shadows with color separation and provide an output with poor color uniformity. For example, a source featuring blue and yellow sources may appear to have a blue tint when viewed head on and a yellow tint when viewed from the side. Thus, one challenge associated with multicolor light sources is good spatial color mixing over the entire range of viewing angles. One known approach to the problem of color mixing is to use a diffuser to scatter light from the various sources.
Another known method to improve color mixing is to reflect or bounce the light off of several surfaces before it is emitted from the lamp. This has the effect of disassociating the emitted light from its initial emission angle. Uniformity typically improves with an increasing number of bounces, but each bounce has an associated optical loss. Some applications use intermediate diffusion mechanisms (e.g., formed diffusers and textured lenses) to mix the various colors of light. Many of these devices are lossy and, thus, improve the color uniformity at the expense of the optical efficiency of the device.
Many current luminaire designs utilize forward-facing LED components with a specular reflector disposed behind the LEDs. One design challenge associated with multi-source luminaires is blending the light from LED sources within the luminaire so that the individual sources are not visible to an observer. Heavily diffusive elements are also used to mix the color spectra from the various sources to achieve a uniform output color profile. To blend the sources and aid in color mixing, heavily diffusive exit windows have been used. However, transmission through such heavily diffusive materials causes significant optical loss.
Some recent designs have incorporated an indirect lighting scheme in which the LEDs or other sources are aimed in a direction other than the intended emission direction. This may be done to encourage the light to interact with internal elements, such as diffusers, for example. One example of an indirect fixture can be found in U.S. Pat. No. 7,722,220 to Van de Ven which is commonly assigned with the present application. Many different types and designs of indirect fixtures are possible.
Modern lighting applications often demand high power LEDs for increased brightness. High power LEDs can draw large currents, generating significant amounts of heat that must be managed. Many systems utilize heat sinks which must be in good thermal contact with the heat-generating light sources. Troffer-style fixtures generally dissipate heat from the back side of the fixture that extends into the plenum. This can present challenges as plenum space decreases in modern structures. Furthermore, the temperature in the plenum area is often several degrees warmer than the room environment below the ceiling, making it more difficult for the heat to escape into the plenum ambient.